1. Field of the Invention
The invention relates to a nuclear reactor installation including core catching apparatus adapted to be disposed below the core of a nuclear reactor for containing the components of a nuclear reactor core during a core melt-down accident.
2. Description of the Prior Art
If in a loss-of-coolant accident during the operation of a nuclear reactor the emergency core cooling system fails, parts of the core may melt as a result of the heat produced during the rapid decay of the fission products formed by fission of the nuclear fuel atoms during operation of the nuclear reactor.
Following a loss-of-coolant accident, a nuclear reactor is rapidly shut down by insertion of all control rods into the reactor core. However, even if the reactor is properly shut down and heat production by nuclear fission has ended the decay of the fission products in the nuclear reactor core may still generate sufficient heat to melt the core if the core emergency cooling system fails to cool the core after reactor shutdown.
How serious such a core melt-down becomes, that is particularly, how long the melt remains liquid thereby forming a threat of destruction for the reactor containment, depends to a great extent on the degree of removal of the fission products from the nuclear fuel. Removal of the fission products from the melt naturally reduces the heat generation in the melt. However, knowledge of the size of any heat sources remaining in the melt is considered to be necessary for the determination of apparatus designed for the protection of the containment such as core catchers. Each core catcher must be designed for accommodation of all the heat that may be generated in a core after reactor shutdown. In accordance with the present state of the art, it is believed that about 65 .+-. 15% of all radioactive heat sources remain in the melt after core melt-down.
It is possible that, as a result of a core melt-down, the reactor vessel bottom is melted and even the concrete foundation of the containment is destroyed so that fission products are released. This possibility, of course, forms a great threat to the environment.
It is furthermore possible, particularly in water-cooled reactors, that exotherm chemical reactions take place between the coolant and the fuel rod cladding or other structural materials which usually result in the formation of hydrogen which, together with the oxygen in the containment, forms a highly explosive mixture which, when ignited, could easily destroy the reactor containment resulting in a widespread radioactive contamination of the environment. However, such chemical reactions could also occur in gas cooled reactors if, for example, a break occurs in the steam generator resulting in a water break-in into the core melt.
To avoid the formation of a critical mass after a core-melt-down-accident, a core catcher has been proposed (German DOS No. 2,259,071) which for example consists of a number of superimposed pan-type containers with a central body having at its upper end a conical head. Below its conical head, the central body has two superposed grid structures which are provided with vanes and which distribute the melt and retain fuel particles of various sizes. The liquid melt passes through the grid structures and is retained in the pan-type containers disposed below the grid structures.
Such core catching containers have the purpose of containing the melt within the reactor containment. The decay heat is carried away partially through the walls of the container and partially from the surface of the melt in the container.
In order to contain the molten core parts within the reactor containment it is not only necessary to remove heat from the melt but also to prevent the radiation heat emerging from the melt from melting or damaging portions of the reactor structures or the containment. To prevent such damages preventative structures may be provided or the injection of water may be taken into consideration.
However, whether water can be used for cooling the surface of the melt depends on the possibility of chemical reactions between the water and the melt such as reactions causing formation of hydrogen. In a water cooled reactor contact of the molten core with water cannot be prevented. In a gas-cooled reactor, such contact between water and the melt may be caused by water from a break in the steam generator or by water injected for cooling the reactor internals and the melt.